The present invention relates to data storage systems, and more particularly, this invention relates to error correction mapping.
Business, science and entertainment applications depend upon computing systems to process and record data. In these applications, large volumes of data are often stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, optical disk cartridges, floppy diskettes, or floptical diskettes. Typically, magnetic tape is the most economical, convenient, and secure means of storing or archiving data.
Storage technology is continually pushed to increase storage capacity and storage reliability. Improvement in data storage densities in magnetic storage media, for example, has resulted from improved medium materials, improved error correction techniques and decreased areal bit sizes. The data capacity of half-inch magnetic tape, for example, is currently measured in hundreds of gigabytes.
The improvement in magnetic medium data storage capacity arises in large part from improvements in the magnetic head assembly used for reading and writing data on the magnetic storage medium. A major improvement in transducer technology arrived with the magnetoresistive (MR) sensor originally developed by the IBM® Corporation. Later sensors using the GMR effect were developed. AMR and GMR sensors transduce magnetic field changes to resistance changes, which are processed to provide digital signals. AMR and GMR sensors offer signal levels higher than those available from conventional inductive read heads for a given read sensor width and so enable smaller reader widths and thus more tracks per inch, and thus higher data storage density. Moreover, the sensor output signal depends only on the instantaneous magnetic field intensity in the storage medium and is independent of the magnetic field time-rate-of-change arising from relative sensor/medium velocity. In operation the magnetic storage medium, such as tape or a magnetic disk surface, is passed over the magnetic read/write (R/W) head assembly for reading data therefrom and writing data thereto.
The quantity of data stored on a magnetic tape may be increased by increasing the number of data tracks across the tape. More tracks are made possible by reducing feature sizes of the readers and writers, such as by using thin-film fabrication techniques and MR sensors. However, the feature sizes of readers and writers cannot be arbitrarily reduced. Factors such as lateral tape motion transients and tape lateral expansion and contraction must be balanced with reader/writer sizes that provide acceptable written tracks and readback signals. One particular problem limiting areal density is misregistration caused by tape lateral expansion and contraction. Tape width can vary by up to about 0.1% due to expansion and contraction caused by changes in humidity, tape tension, temperature, etc.
Thus, while the reader/writer array width does not change, the spacing of the data tracks on the tape will vary as the tape expands and contracts. Ideally, the reader track width would be as wide as the data track being read; this would provide the best signal. However, sensor track widths cannot be made as wide as the data tracks, because the sensors would read adjacent tracks upon expansion or contraction of the tape and/or due to lateral misregistration between tape and head. Accordingly, reader widths are currently designed to be substantially smaller than the data track width, and all readers in a given head having the same track width. The reader track width is selected to accommodate the worst case scenarios, i.e., the designer takes into account maximum expansion/contraction and lateral misregistration when determining reader track width so that each sensor is over a given track at any time. FIGS. 1 and 2A-2B represent the effect of tape lateral expansion and contraction on reader position relative thereto. FIG. 1 shows the head 100 relative to the tape 102, where the tape has a nominal width. As shown, the readers 104 are aligned with the data tracks 106 on the tape 102. FIG. 2A shows the effect of tape lateral contraction. As shown, the outermost readers 108 are positioned along the outer edges of the outer data tracks. FIG. 2B shows the effect of tape lateral expansion. As shown, the outermost readers 108 are positioned along the inner edges of the outer data tracks. Because all of the readers 104 have the same width, the readback signal level from each reader will normally be the same.
In any kind of magnetic recording, error detection and correction has great practical importance in maintaining data (information) integrity across noisy channels and potentially unreliable storage media. An error-correcting code (ECC) or forward error correction (FEC) code is a code in which each data signal conforms to specific rules of construction so that departures from this construction in the received signal can generally be automatically detected and corrected. It is used in computer data storage, for example in magnetic recording, dynamic RAM, and in data transmission.
ECC processing is currently applied to all tracks equally. However, such uniform partitioning is inefficient both in terms of the amount of media space to store the code as well as power to drive the ECC processing.
Moreover, there is also a need to increase data storage capacity for a given area of media.